TUI backend:
- Add ratatui-image for native terminal image display (Kitty, iTerm2,
  Sixel protocols with Unicode halfblock fallback)
- Load images from HTTP URLs (ureq), base64 data URIs, and local files
- Graceful fallback to [Image: alt] placeholder when unsupported
- Images pre-loaded at parse time, rendered inline in scrollable content

Unit tests (58 total):
- core/toc: slugify (8 tests), extract_toc (9 tests)
- core/markdown: heading IDs (4), strip_html (1), parse_markdown (8)
- core/mermaid: html encode/decode (6), render (4), process blocks (4),
  egui preprocessing (3)
- backend/egui: split_by_headings (10 tests)

Also: resolve local image paths to absolute file:// URLs in egui backend

prompt: TUI image rendering with protocol auto-detection, comprehensive
unit tests for all core modules, local file image resolution

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>